1. Field of Invention
This invention relates to a three degree-of-freedom positioning and manipulating apparatus, and, more particularly, to a three degree-of-freedom manipulating apparatus constructed from three rotary drives.
2. Description of the Prior Art
Three degree-of-freedom manipulators as commonly employed in industrial robots generally follow one of two design methods. The first method is shown in FIG. 1. It employs a rotatable base 80, to which is mounted a first arm 20 which can tilt at a controlled angle relative to the base 80. A second arm 40 is mounted to the end of the first arm 20 and can swing at a controlled angle relative to the first arm 20. To reduce inertia, drive motor 60 for the second arm 40 is usually located in line with the joint between the first arm 20 and the base 80 coaxial with drive motor 50. Belts or chains (not shown) are required to deliver torque to rotate the second arm. This introduces compliance and friction in the joint, reducing its positioning accuracy and the speed of its response. The additional components also increase the mass and hence the inertial load on the motors. The masses of drive motors 50 and 60, located outside of the axis of rotation of the base 80, increase the inertia seen by the base 80.
The second design approach, as shown in FIG. 2, also employs a rotating base. Mounted to the base 80 is a four bar linkage 82 which provides an additional two degrees of freedom. Motor 50 controls the relative angle of the linkage 82 to the base 80, while motor 60 controls the angle between the two pairs of links. The motor placement helps to minimize inertia, as the motors do not move with the links. The multiple rigid links improves stiffness over a belted design, however they have more mass and thus contribute greater inertia. With four joints in the linkage, friction will be higher and thus accuracy reduced.
For both of these designs, loads at the end of the long links create large inertias. To control these inertias without using exceptionally large motors, transmission reduction mechanisms are added to increase drive torque. These mechanisms typically employ a system of gears, which contribute additional mass, friction, compliance and roughness.
Thus, there is a strong need in the art for a three degree-of-freedom manipulator with reduced friction, inertia, mass, etc.